High sensitivity magnetometers, including paramagnetic resonance magnetometers (PRM) (Slocum & Reilly, 1963), have a wide range of applications including, but not limited to the following: fundamental research, detecting biomagnetic signals (such as those emanating from biological organisms, including the human body), geophysical exploration and prospecting, navigation and space applications, and military uses (such as ordinance and underground-underwater structure detection). Until recently, the most sensitive and commercially available magnetometers for such applications were based on superconducting quantum interference devices (SQUID) (Weinstock, 1996). However, zero-field paramagnetic resonance magnetometers (ZF-PRM); (Dupont-Roc, Haroche, & Cohen-Tannoudji, 1969; Marie-Anne et al., 1971; Shah, Knappe, Schwindt, & Kitching, 2007; W & E, 1974), which have advanced to comparable sensitivity as SQUID systems, have recently gained popularity as a lower-cost, more robust alternative to SQUID magnetometers for many applications. The current rapid development and commercialization of such atomic based magnetic sensors may lead to replacement of SQUID based sensors for many existing applications primarily because ZF-PRMs do not require cryogenic cooling.
Significant developments in alkali atomic magnetometery (Budker & Romalis, 2007) over the past decade have led to a variety of techniques and methods for sensing magnetic fields. In general, the different methods are based on the same fundamental physical sensing mechanism that exploits the energy structure of atoms and the perturbations that result in their energy levels (or spin state) from exposure to external magnetic fields. In essence, atomic based magnetic sensors measure the direction and magnitude of an external magnetic field through the induced changes in the atomic spin polarization of an ensemble of atoms.
A ZF-PRM relies on detecting changes in optical transmission properties of atomic vapor around a narrow ZF atomic resonance to measure the magnitude and direction of the background magnetic field. Generally a ZF resonance can only be observed when the atomic vapor (the ensemble of atoms) in the magnetometer is subjected to very small magnetic fields, generally less than 100 nanotesla (nT). Typically, the full width of the ZF resonance is less than 30 nT. It is therefore necessary for the ambient magnetic field at the location of ZF-PRM to be less than the detection range of the magnetometer. The detection range of a ZF-PRM is typically a fraction (less than about 1, for example, one half) of the width of the ZF resonance. The magnetometer becomes less sensitive or even insensitive when the total background magnetic field is greater than the detection range. For this reason, the ZF-PRMs are frequently used inside magnetically shielded environments in which the ambient magnetic field is very small, typically few tens of nT or less. When ZF-PRM is used in an unshielded or poorly shielded environment where a large background magnetic field is present, external biasing coils are used to null the magnetic field in vicinity of the ZF-PRM, keeping the magnetometer within its detection range. This operational requirement of near ZF condition limits the utility of ZF-PRMs for many applications; this includes biomedical applications, such as Magnetoencephalography (MEG) and Magnetocardiography (MCG) where large and expensive shielded volumes are required, as well as any outdoor applications where magnetic shielding is impractical.